Widely-tuned semiconductor laser based gas liquid solid analysis system

ABSTRACT

A near infrared (NIR) semiconductor laser system is shown for gas sensing. An embodiment is centered on the use of a system with a much wider tunable laser, which today has a scan band of more than 150 nanometers (nm) to as much as 250 nm or more. In some cases the scan band is about 400 nm or more. This is achieved in the current embodiment through the use of a widely tunable microelectromechanical system (MEMS) based Fabry-Perot filter as an integral part of the laser cavity. Using this technology, these systems are capable of capturing a variety of gases in the any of the well-known spectroscopic scan bands, such as the OH, NH or CH. For example, a single laser with a 250 nm scan band window between 1550-1800 nm can capture ten or as many as twenty hydrocarbon-based gases simultaneously.

RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. application Ser. No.11/948,520, filed on Nov. 30, 2007, which claims the benefit under 35USC 119(e) of U.S. Provisional Application No. 60/867,849 filed on Nov.30, 2006, both of which are incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

Combustible gas detectors can be divided into two general categories.The first category includes a variety of “passive” technologies of whichthe electrocatalytic (catalytic bead) type is the most common. Thesecond category is based on technology that uses infrared absorption asthe detection modality.

Infrared (IR) absorption is considered “active” since an IR source emitsa signal, usually many times a second, and the amount of energy fallingon the detector serves as an active measure of the gas concentration atthat moment. Any failure of the source or detector, or blockage of thesignal by dirt, is detected immediately as a malfunction. For thisreason, IR detectors are also considered to be fail-to-safe. IR gasdetectors can be used for “point” (single location) or “open path” (lineof sight) applications.

Electrocatalytic or “catalytic” detectors have been used for over 30years and are widely deployed in a variety of industries as single-pointdetectors for combustible gases. They function on the relatively simpleand reliable principle that a combustible gas can be oxidized to produceheat. The resulting temperature change can be converted, via a standardWheatstone bridge, to a sensor signal. That signal can then be used toactivate alarms and initiate fire preventative action. Even though thesedetectors can be manufactured to be very low cost, their primarydrawback is that they can be contaminated or “poisoned” and they hencerequire dedicated user attention.

An alternative method of measuring gas concentration is based onabsorption of infrared (IR) radiation at certain wavelengths as theoptical signal passes through a volume of gas. Devices using thistechnology have a light source and a light detector and measure thelight intensity at two specific wavelengths, one at an absorption(active) wavelength and one outside of the absorption (reference)wavelength. If a volume of gas passes between the source and detector,the amount of light in the active wavelength falling on the detector isreduced, while the amount of light in the reference wavelength remainsunchanged. Much like the catalytic detectors, the gas concentration isdetermined from the relative difference between the two signals.

Generally, there are several key advantages to using IR-based detectors:immunity from chemical poisons; does not need oxygen or air to detectgas; can work in continuous exposure gas environments; fail-to-safetechnology; and internal compensation can eliminate span drift.

SUMMARY OF THE INVENTION

Embodiments of the present invention concern the use of Near Infrared(NIR) semiconductor lasers for gas sensing. This technology is expectedto address some of the limitations of the current IR detectors. Suchlimitations include the following: 1) interference from other gases ormoisture, due to the mid-IR operation as well as the fact that these IRdetectors are typically low resolution and cover a limited spectralrange, which prevents them from being able to identify and correct forthe interfering gases; 2) inability to detect multiple gases with asingle device, since these detectors use fairly coarse optical filtersover a specific spectral range and therefore cannot detect gases withspectral signatures outside that window; and 3) limited sensitivity dueto low signal-to-noise ratio, which is primarily attributed to the useof mid-IR sensitive detectors (pyroelectric), which are typically not assensitive as InGaAs detectors.

The key elements of embodiments of the invention often include a numberof aspects.

The use of a widely-tuned semiconductor laser is important. Currentlythere are laser-based gas detectors that use a technology called TunableDiode Laser Absorption Spectroscopy (TDLAS), but these devices are verynarrow in spectral range (10-60 nanometers (nm)) and they use adithering technique to monitor individual gases. It is not possible touse these devices to monitor multiple gases. An embodiment of theinvention is centered on the use of a system with a much wider tunablelaser, which today has a scan band of more than 150 nm to as much as 250nm or more. In some cases the scan band is about 400 nm or more. This isachieved in the current embodiment through the use of a widely tunablemicroelectromechanical system (MEMS) based Fabry-Perot filter as anintegral part of the laser cavity. Using this technology, these systemsare capable of capturing a variety of gases in the any of the well-knownspectroscopic scan bands, such as the OH, NH or CH. For example, asingle laser with a 250 nm scan band window between 1550-1800 nm cancapture ten or as many as twenty hydrocarbon-based gases simultaneously.

High resolution spectroscopy is provided by use of the MEMS Fabry-Perotfilter, which enables the potential for operation at very highresolution (as high as 0.1 cm⁻¹), which allows the system to identifyfine spectral features of the interfering gases (such as moisture) andcorrect for them, providing a much more accurate reading.

High sensitivity measurements are further enabled, providingapplicability beyond the detection of explosive or flammable gases, tobe used for any gas, liquid or solid spectral measurement, assuming thatthe gas, liquid or solid in question absorbs infrared radiation.

The preferred embodiment also provides for on-board referencing. Alsothe higher SNR of embodiments leads to higher measurement speed andhence there is a tradeoff option between laser power, detectorsensitivity and measurement speed depending on the applicationrequirements. Further small explosion proof packaging is preferablyprovided: The use of miniaturized packages allows for reducedrequirements on the size of the explosion proof housing. Also, thehighly efficient NIR semiconductor lasers allow for minimal powerconsumption and potentially improving battery life, if used. In thepreferred embodiment, the system includes wireless communication.

In general, according to one aspect, the invention features aspectroscopy system. The system comprises a hermetic package with atleast one optical port for coupling an optical beam from the hermeticpackage to a sample and back into the hermetic package. An optical benchis installed within the hermetic package, and a tunable laser isinstalled on the optical bench for generating the optical beam. A signaldetector is further provided on the bench for detecting the optical beamreceived in through the optical port from the sample. Also, a controllerspectrally analyzes the sample from a response of the signal detector.

In embodiments, a housing contains the hermetic package, the housingcomprises a sample port for receiving the sample and the optical beamfrom the hermetic package into the sample port. A mirror is preferablyinstalled in the sample port for reflecting the optical beam back intothe optical port.

Additional possible features include a battery for powering thecontroller and the tunable laser and a wireless interface for providingwireless communication of spectral analysis information from thecontroller.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a perspective view of a system for gas detection and analysisaccording to an embodiment of the present invention;

FIG. 2 is a perspective view of the system for gas detection andanalysis showing the optical bench layout according to an embodiment ofthe present invention;

FIG. 3 is a block diagram view of the system for gas detection andanalysis showing relationship between the optical bench layout andsystem electronics according to an embodiment of the present invention;

FIGS. 4A, 4B, and 4C show three variations for optical interface to thegas sample port, single bounce, double bounce and triple bounce,according to embodiments of the present invention;

FIG. 5 is a plot of absorbance as a function of wavelength in nanometersfor 100% methane measured by the system;

FIG. 6 is a plot of absorbance as a function of wavelength in nanometersfor 0.4% methane measured by the system;

FIG. 7 is a plot of absorbance as a function of wavelength in nanometersfor 0.2% ethylene measured by the system;

FIG. 8 is a plot of concentration as a function of time in measurementcycle for a methane sample measured by the system; and

FIG. 9 illustrates is a schematic cross-sectional view of the gas sensorsystem 100 illustrating the thermal dissipation scheme according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a system 100 for gas detection and analysis, which has beenconstructed according to the principles of the present invention.

Generally, detection of several gases that have spectroscopic signaturesin the near-infrared zone of 1.3-2 micrometers are detected by using asingle-chip tunable source covering 200-300 nanometer (nm) scan band inthis zone. Such gases include most of the common hydrocarbons, such asmethane, ethane, ethylene, acetylene and other types, such as ammonia.The detection limits for the system are preferably in the 10% of lowerexplosion limit (LEL) levels.

The gas sensor system 100 preferably comprises three basic functionalcomponents: an optical module 110, an electronics board 250, and a gassample optical interface.

The optical module 110 comprises a single hermetically sealed butterflypackage 112, which contains all of the active optical functions of thesystem 100 assembled on a single micro-optical bench 105: a widelytunable semiconductor laser, optical amplitude and wavelength referencedetectors, and the signal detector.

The optical module 110 produces a collimated optical beam output 16 intothe gas sample optical interface. Specifically, this beam 16 interactswith the sample gas in an optical sample interface port 210 in a housing200, using one or more bounces in an external mirror arrangement inorder to accumulate a sufficient optical absorption path length.

The returning beam 18 from the gas sample optical interface is detectedby the signal detector inside the optical module 110.

The beam exits and enters the hermetic package 112 via at least oneoptical port 116, 118, comprising at least one window being transparentto the optical beam. This window is formed as part of an outer wall ofthe hermetic package 112.

The electronics board 250 contains analog and digital components forcontrolling tunable laser scanning, detecting and digitally processingthe return signal, and outputting the gas sensing data in the digitalformat. In some examples, external system control and data upload areprovided via a wireless interface 254. Also, in examples, the system 100includes a battery 252 for main system power and/or backup power.

In more detail, the system housing 200 comprises a base plate 216. Aframe unit 214 is installed and sealed to this base plate 216.Generally, the frame unit 214 of the housing 200 provides a hollowinternal area in which the optical module 110 and specifically thebutterfly package 112 is installed. The hollow internal area furtherhouses the wireless interface 254, the battery 252 and the digitalsignal processing electronics board 250. A top cap 212 is sealed to theframe member 214 to seal the internal area and thus the electronic andoptical components.

A central port 212 is formed in the housing 200 and specifically theframe member 214, base plate 216 and the top member 212. The opticalsample interface port 210 mechanically interfaces with the outputoptical port 216 and the input optical port 218, which are formed on theside wall of the butterfly package 212. At the far lateral end of theoptical sample interface port 210, a mirror 114 is provided to couplethe output beam 116 as the input beam 118 between the output opticalport 116 and the input optical port 118.

In the preferred embodiment, the output optical port 116 and the inputoptical port 118 comprise optically transparent windows. In someexamples, these windows are bandpass filters that are limited to thescan band of the system 100 to thereby to prevent interference fromexternal light sources.

While the applicability of the system is described in the context of gasanalysis, the system 100 also has application in fluid and solidanalysis. In some of these applications, the system uses the mirror 114where the gas, liquid or solid sample is transmissive. When it is opaqueto the wavelengths within the scan band, system collects the diffuselyreflected light from the sample for analysis.

FIG. 2 illustrates the internal optical components 111 of the gas sensoroptical module 110 within the hermetic package 112 including themicro-optical bench layout.

The widely tunable external cavity semiconductor laser comprises asemiconductor gain chip 120 which is installed on the optical bench 105via a submount substrate 122. A micro-electro-mechanical (MEMS) tunableoptical filter 126 provides narrowband tunable feedback into the gainchip 120. An intervening intracavity lens component 124 couples lightbetween the chip 120 and the MEMS filter 126.

In a current embodiment, the laser is as described in US 20060215713 A1,entitled Laser with Tilted Multi Spatial Mode Resonator Tuning Element,by Flanders, et al. This application is incorporated herein in itsentirety by this reference.

A sample of the output beam 16 is provided from a collimating lens 128to two taps 130, 132 to an amplitude detector 140 and a wavelengthreference detector 138. For wavelength referencing, a fixed wavelengthetalon 134 and a possible a cut-off filter 136 are placed in front ofthe wavelength detector 138 to provide wavelength calibration referencefor the wavelength scanning output laser signal, along with theamplitude information provided by the amplitude detector 140.

A combination of two series output lens 142 and 144 are used for bothcollimating the output beam 16, received from the collimating lenscomponent 128 and the first tap 130, at the proper beam diameter and forfinely steering the beam 16 for alignment to the optical gas samplinginterface outside the optical package 112. Specifically, the beam issteered to be coupled back into the package 112 via input port 118 anddetected by signal detector 150.

The return optical signal detector 150 is integrated on the samemicro-optical bench 105.

In the preferred embodiment, the optical bench 105 is temperaturecontrolled. This is provided by a thermoelectric cooler 106.

The requirements for explosive gas monitoring are: that the systems arecapable of detecting down to 10% of the Lower Explosive Limit (LEL) foreach gas. For methane, this limit is 5%, so the detectors must beaccurate to 0.5%. However, regulatory and safety requirements arepushing these limits down, and there is a need for higher sensitivitydevices. Operation at the near infrared (NIR) regime for the presentsystem 100 allows for the use of more sensitive detectors, such asInGaAs rather than pyroelectric, as it is currently the case withexisting IR sensors. In addition, the use of semiconductor, high powerlasers allows for higher signal-to-noise ratio measurements, whichdirectly translates to lower sensitivity limits.

FIG. 3 illustrates the relationship between the electronics and theoptical bench 105 in a current embodiment of the invention.Specifically, a digital signal processing board 150 controls power tothe gain chip 120 and the tuning of the tunable filter 126. Digitalsignal board 250 receives the response of the signal detector 150.

By analyzing the response of the signal detector in conjunction with theinformation from the wavelength detector 138 and the amplitude detector140, the digital signal processor 250 resolves the spectral response ofthe gas sample within the gas port. Further, the digital signalprocessor 250 monitors the temperature of the optical bench 105 via atemperature detector 160, such as a thermistor, which is typicallyinstalled on the surface of the bench 105. Using this feedback, thedigital signal processor board 250 drives the thermoelectric cooler 106to temperature stabilize the optical bench 105.

In one embodiment, the system is also provided with a battery 252 forpower. Further, a wireless interface 254 is used in some examples toboth provide control to the spectral analysis system 110 and alsoprovide data upload to a host system.

FIGS. 4A through 4C illustrate a number of examples for configuring thebeams in the gas interface port 210. FIG. 4A shows one example where theoutput beam 16 is reflected to form the input beam 18 by the mirror 114in a single bounce arrangement.

In FIG. 4B, a double bounce arrangement is used in which a second mirror114′ is added optically between the output port 118 and the input port116 of the hermetic package 112. Specifically, this doubles the beampath length by yielding passes 117 through port 210.

FIG. 4C illustrates a triple bounce arrangement in which the output beam16 is reflected between mirrors 114 and 114′ three times beforereturning to the input port 118.

FIGS. 5 and 6 are plots of absorbance as function of wavelength innanometers for methane gas sensing spectra at 100% and 0.4%concentration levels using a single pass 10 centimeter (cm) absorptioncell, which were obtained using the system 100 for gas detection andanalysis. The methane Lower Explosive Limit (LEL) is 5.0% and therequired sensing level of 10% LEL is 0.5%. High signal to noise ratiosof the spectra shows that methane can be sensed to very lowconcentrations.

FIG. 7 shows a similar gas-sensing spectrum for ethylene gas in a 10 cmpath cell, illustrating simple sensing of different gases using thespectroscopic gas system. Such system also potentially allowsmeasurement of mixtures of multiple gases.

FIG. 8 illustrates concentration measurements of methane gas in a 10 cmpath cell using gas sensor 100, where gas concentration number isextracted from the measured spectra. Methane gas concentration ismeasured down to the 0.15% level, which is equivalent to the 0.03*LELlevel. Gas detection sensitivity can be enhanced even further by using avery compact multi-pass gas cell allowed by the single transverse modeoptical beam of the sensor.

FIG. 9 illustrates the thermal dissipation paths of the gas sensorsystem 100 that prevent the optical ports 116, 118 and mirror 114 fromaccumulating condensation.

In the preferred embodiment, the optical ports 116, 118 and/or mirror114 are held at temperatures above ambient temperature. As a result,even in highly saturated environments, condensation will be preventedfrom forming on the optical ports 116, 118 and/or mirror 114.

Elevated temperatures for the optical ports 116, 118 and/or mirror 114are achieved in one embodiment by including heaters near or on theoptical ports 116, 118 and/or the mirror 114. Simple resistive heatersare used in some examples.

However, in the preferred embodiments, the thermal dissipation paths forthe optical system 111 and/or electronic system 250 are judiciouslydesigned to provide these elevated temperatures.

In one example, the thermal dissipation path for heat generated by thethermoelectric cooler 106 is designed to extend through to the lid 112Lof the hermetic package 112. In more detail, the thermoelectric cooler106 has a hot side 912 and cool side 910. The hot side 912 is thermallycoupled to the bottom 112B of the hermetic package 112 as isconventional. However, a thermally insulating layer or air or vacuum 930separates the bottom 112B of the package 112 from a thermal sink such asthe baseplate 216. Instead, the thermal dissipation path is through thepackage 112 to the lid 112L. In one example, a thermally conductivepaste is used to create a thermal path from the lid 112L to the cap 212.This thermal dissipation path through the package 112 and thus the ports116, 118 of the package 112 ensures a heated state of the ports 116, 118during operation of the system 100.

In a similar way, the mirror 114 is also included in a thermaldissipation path. In more detail, in one example, heat generated by theelectronics of the DSP board 250 is dissipated through the portion ofthe frame member 214 that supports the mirror 114 to a thermal sink 920.As a result, the mirror 114 is passively heated by the “waste” heatgenerated by the electronics 250 to ensure that the mirror 114 is hotterthan the environment during operation, thereby preventing theaccumulation of condensation on the mirror 114.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the invention.

1. A spectroscopy system, comprising a hermetic package with at leastone optical port for coupling an optical beam from the hermetic packageto a sample located outside of the hermetic package and back into thehermetic package; an optical bench within the hermetic package; atunable laser on the optical bench for generating the optical beam; anda signal detector on the bench for detecting the optical beam receivedin through the optical port from the sample; and a controller forspectrally analyzing the sample from a response of the signal detector.2. A system as claimed in claim 1, further comprising a housingcontaining the hermetic package, the housing comprising a sample portfor receiving the sample and the optical beam from the hermetic packageinto the sample port.
 3. A system as claimed in claim 2, furthercomprising a mirror in the sample port for reflecting the optical beamback into the optical port.
 4. A system as claimed in claim 1, furthercomprising a battery for powering the controller and the tunable laser.5. A system as claimed in claim 1, further comprising a wirelessinterface for providing wireless communication of information from thespectral analysis from the controller.
 6. A system as claimed in claim5, further comprising a battery for powering the controller and thetunable laser.
 7. A system as claimed in claim 6, further comprising ahousing containing the hermetic package, battery and wireless interface,the housing comprising a sample port for receiving the sample and theoptical beam from the hermetic package into the sample port.
 8. A systemas claimed in claim 1, further comprising a thermoelectric cooler forcontrolling a temperature of the optical bench.
 9. A system as claimedin claim 1, wherein the at least one optical port comprises a windowbeing transparent to the optical beam which is functions as part of anouter wall of the hermetic package.
 10. A system as claimed in claim 1,wherein a wavelength of the optical beam is tuned over a scan bandextending over 150 nanometers.
 11. A system as claimed in claim 1,wherein a wavelength of the optical beam is tuned over a scan bandextending over 250 nanometers.
 12. A system as claimed in claim 1,wherein a wavelength of the optical beam is tuned over a scan bandincluding the band extending from 1550 to 1800 nanometers.
 13. A systemas claimed in claim 1, wherein a wavelength of the optical beam is tunedover a scan band including the band extending from 1300 to 2000nanometers.
 14. A system as claimed in claim 1, wherein the controllerassesses gas concentrations relative to a lower explosion limit forthose gases.
 15. A system as claimed in claim 1, further comprising awavelength reference detector on the optical bench for detecting aportion of the optical beam to determine a wavelength.
 16. A system asclaimed in claim 1, further comprising an amplitude detector on theoptical bench for detecting a portion of the optical beam to determinean amplitude.
 17. An optical analysis system, comprising a hermeticpackage with at least one optical port for coupling a tunable opticalsignal from the hermetic package to a sample located outside of thehermetic package and back into the hermetic package, the at least oneoptical port comprising a window being transparent to the tunableoptical signal and functioning as part of an outer wall of the hermeticpackage; an optical bench within the hermetic package; a tunable opticalsource on the optical bench for generating the tunable optical signal;and a signal detector on the bench for detecting the tunable opticalsignal received in through the optical port from the sample; and acontroller for analyzing the sample from a response of the signaldetector.
 18. A system as claimed in claim 17, further comprising athermoelectric cooler for controlling a temperature of the opticalbench.
 19. A system as claimed in claim 17, wherein the at least oneoptical port comprises a window being transparent to the optical beamand which functions as part of an outer wall of the hermetic package.20. A system as claimed in claim 17, wherein a wavelength of the tunableoptical signal is tuned over a scan band extending over 150 nanometers.21. A system as claimed in claim 17, further comprising a port heatsource for heating the at least one optical port.
 22. A system asclaimed in claim 21, further a thermoelectric cooler for controlling atemperature the tunable optical source and the signal detector andfunctioning as the port heat source.